1. Field of the Invention
The invention relates to a method of manufacturing a planar waveguide using an ion exchange method, and more particularly to a method of manufacturing a planar waveguide formed in a glass substrate having a step wall. The method of this invention can precisely control the dimension of waveguide and has an outstanding reproducibility.
2. Description of the Prior Art
Recently, as wavelength division multiplexing (WDM) optical communication systems are employed, demand for optical devices used in the WDM optical communication systems has been significantly increased.
Optical fiber is usually used as a transfer path of optical signals on optical communications. However, there is a technical limit in manufacturing the fiber device having multi-channel such as a multi-channel optical coupler and WDM device. Therefore, a planar light waveguide circuit (PLC) where optical waveguides and many unit optical devices are integrated is used in WDM devices. The planar type optical devices can be made of various materials such as glass, semiconductors, amorphous silica and polymer. Amorphous silica is similar to material for the optical fiber and has low transmission loss and low coupling loss with the optical fiber. The PLC made by amorphous silica can be manufactured with a similar process to the semiconductor integrated circuit manufacturing process, therefore, it has been widely used as materials for the planar optical waveguides. A method of manufacturing the planar waveguides using amorphous silica includes a flame hydrolysis deposition (FHD) and a reactive ion etching (RIE) method. The silica planar waveguide produced by the FHD and RIE method has a relatively high accuracy of waveguide dimensions and has a sharp step wall of waveguide. However, the FHD process requires a high temperature, and it is very difficult to control the processing factors. In addition, the core portion of the waveguide may be changed by high temperature process when the surface cladding layer is stacked, which makes the quality control difficult and results in an increase of the manufacturing cost.
Due to these problems, a method of manufacturing the planar waveguides by means of an ion exchange method using special silicate glass, which is simple in the manufacturing process, has recently been developed.
An example for the method of manufacturing the planar waveguide using the ion exchange method includes U.S. Pat. No. 4,913,717 (Apr. 3, 1990) entitled xe2x80x9cMethod for Fabricating Buried Waveguides, U.S. Pat. No. 5,035,734 (Jul. 31, 1991) entitled xe2x80x9cMethod of Producing Optical Waveguidesxe2x80x9d, U.S. Pat. No. 5,160,360 (Nov. 3, 1992) entitled xe2x80x9cProcess for Producing Low-loss Embedded Waveguidexe2x80x9d, Strip Waveguides with Matched Refractive Index Profiles Fabricated by Ion Exchange in Glass (pp.1966xcx9c1974 of xe2x80x98J. Appl. Physxe2x80x99, 1991 by T. Poszner, G. Schreiter, R. Muller), Field-Assisted Ion Exchange in Glass: the Effect of Masking Film (pp.1212xcx9c1214 of xe2x80x98Appl. Phys. Lettxe2x80x99, 1993 by B. Pantchev, P. Danesh, Z. Nikolov), and Polarization Insensitive Ion-Exchanged Arrayed-Waveguide Grating Multiplexers in Glass (pp. 279xcx9c298 of xe2x80x98Fiber and integrated opiticsxe2x80x99, 1998 by B. Buchold, C. Glingener, D. Culemann, E. Voges).
The method of manufacturing the planar waveguide using the ion exchange method, which was proposed by the patents and papers, can be summarized as follows.
The ion exchange is occurred between a specific ion within the substrate glass (dominantly Na+) and that within a salt solution containing specific ions (such as K+, Ag+, Cs+, Li+, Rb+, and Tl+) when the glass surface between metal thin films called as a mask contacts with the salt solution. Based on this principle, a waveguide having high refractive indices is formed at predetermined portion of the substrate glass.
However, this ion exchange method has disadvantages that it can not exactly control the dimension of the waveguide and does not form the exact difference of refractive index between core and cladding since it basically uses diffusion of specific ions such as Na+, K+, Ag+. In case of silicate glass, a network structure of Si4+ ion and oxygen ion has a strong covalent bond, but alkali ions have relatively a week bonding with oxygen ions and exist at the vacant places between oxygen ions. Therefore, when the glass surface is contacted with molten salt at high temperature, a part of Na+ ion having a small ionic radius in the glass is diffused out of the glass and K+ or Ag+ ions within the molten salt are diffused into the glass, thus the ions are exchanged. Because a thermal diffusion that governs the ion exchange has no directional characteristics and the driving force of it is determined by the concentration gradient, the concentration profile of exchanged ions in the glass has a gradient distribution from the exchange center. Due to this, the ion exchange method by thermal process is very difficult to form a waveguide having a definite step in the refractive index between core and cladding, a sharp wall, and a precise dimension.
When an electric field assisted ion exchange method is used, exchanged ions are dominantly moved toward a direction to which an electric field is applied, that is, to the cathode. Thus, the electric field assisted ion exchange method can easily control the dimension of the waveguide and can pull down the half circle shaped ion exchange layer on the surface into the glass. However the mobility of network modifier ions in glass is too low at room temperature, it must be heated up to high temperature of 300xcx9c400xc2x0 C. in order to form a desired waveguide. Thus, the electric field assisted ion exchange method is difficult to exactly control the waveguide width and shape and can not avoid a distribution of the refractive index since the diffusion toward a direction vertical to the electric field is not absolutely precluded. In addition, the diffusion distance and the concentration gradient are changed depending on the width of mask aperture and initial ion concentration of the ion exchanged layer even if the ion exchange is performed at same temperature and electric field. Therefore, the conventional electric field assisted ion exchange method is also difficult to manufacture an optical waveguide having a complicate shape such as AWG (arrayed waveguide grating).
The manufacturing process of waveguides using the conventional ion exchange method is performed at a relatively low temperature compared to the FHD-RIE method and employs silicate glass as a substrate, thus the manufacturing cost and time can be reduced. The conventional manufacturing process of waveguides using the ion exchange method has an advantage for mass-production of waveguides, that is, stable, inexpensive and highly durable planar waveguides can be produced. But the conventional ion exchange method has some problems. The metal thin film used as mask is eroded by the reaction between the metal thin film and the molten salt. Also, the dimension or the shape of the waveguide pattern may be changed when the mask of the metal thin film is partially taken off in the process. Further, it takes a lot of time to form the waveguide having a sufficient depth from the surface. It is difficult to form the waveguide wall having a step shape with a sharp step of the refractive index. The waveguide having a complicate shape is difficult to be manufactured because the ion concentration and the penetration depth depend on the waveguide pattern width. There has been an effort to control the amount and depth of ion exchanged by using a mesh-type mask. However, this method is also difficult to precisely control the dimension of waveguides to the accuracy that is required in an optical waveguide.
The object of the present invention is to provide a method that solves the problem of conventional ion exchange method for planar waveguides and produces the ion exchange planar waveguide, which is stable, low cost and has a precisely controlled dimension, good reproducibility and a sharp step wall. In order to accomplish this object, the method of manufacturing the planar waveguide according to the present invention comprises three steps. The firs step is making a surface layer having a higher refractive index than that of glass substrate and a given thickness on a glass substrate by an ion exchange process; the second step is forming the pattern of the waveguide within the surface layer on the glass substrate; and the third step is coating a cladding layer on the entire surface including the waveguide. An ion exchange process can increase the refractive index of the surface layer on the glass substrate. The ion exchange process comprises the steps of dipping the glass substrate into a molten salt for the determined time so that the refractive index of the surface layer of the glass substrate is raised by the ion exchange, and taking out the glass substrate from the molten salt and cleaning the glass substrate.